Flashover prevention means for high-tension ignition apparatus



J. J. ROSE Dec. 19, 1944.

FLASH-OVER PREVENTION MEANS FOR HIGH-TENSION IGNITION APPARATUS Filed Sept. 16. 1942 /Afvavvrok e/o/wv d. f? ss Persmap 19,1944

' FLASIIOVER PREVENTION MEANS Fon l HIGH-TEN SION IGNI'IION APPARATUS John J. Rose, Dayton, Ohio Application September 16, 1942, Serial No. 458,583

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 9 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to Van improved elec.

trical insulation of shielded spark plugs and the terminals therefor, or the connection of the ignition cable to the magneto distributor block, so that the liability of flash-over to ground, when such apparatus is employed on aircraft engines at high altitude, is substantially eliminated. It is well known in the art that at high altitudes flash-over of the high-tension ignition current most frequently occurs in the shielding well of the spark plug or in the corresponding well at the connection of thev ignition cable to the magneto distributor block and is due to the fact that reduced atmospheric pressure at these points allows the spark to jump over a considerable distance to the plug shell or to the grounded shielding elements which' causes a cutting out of, the ignition within the engine cylinder.

Various remedies have been suggested for correcting this condition, one remedyrbeing to increase lthelength of the path from the hightension contacts withinthe magneto distributor block or within the shielding well of the spark plug to such an extent that the spark can not jump to grounded metal elements within the vicinity of said contact. However, increasingI the length of the path to ground requires making the. overall length of the shielding well of a spark plug as muchas six or seven inches, and obviously this remedy is not feasible because of installation difculties of such plugs. A second remedy is to supercharge the distributor, ignition harness and the spark plug shielding wells by connecting the same to the output side of the engine supercharger or special charging pump, which solution, however, requires special sealing provisions in the harness and plugs to prevent excessive air leakage, and further except for the distributor requires special attention from the servicing standpoint.

In according with the present invention a very simple means is employed to prevent flash-over from the high-tession terminals by providing insulating barriers to electrical leakage at each end of the terminal shielding well to adjacent grounded metallic parts'. This electrical sealing of the high-tension contact requires only a simple modification of the present type of plug, or distributor block connection, when used in conJunction with an improved terminal construction described and claimed in my United States Patent No. 2,280,855.

It is, therefore, the object of this invention to improve the electrical insulation of the shielding well of aviation spark plugs and the connection of the ignition cable to the distributor block of the magneto to prevent flash-over of the hightension vignition current ground which such apparatus is employed on high-altitude aircraft.

It is a further object of the invention to completely surround the terminals of ahigh-tension ignition cable by an insulating barrier which will effectively prevent the flash-over of high-tension ignition current from the terminals to adjacent grounded metal objects.

Other objects of the invention will appear by reference to the detailed description in the speciilcation hereinafter given, and to the appended drawing in which:

Figure 1 is a sectional view illustrating the construction of a conventional shielded spark plug now widely employed for aviation engine ignition purposes;

Figure 2 is a view partly in section illustrating to an enlarged scale the construction of a cable terminal employed as a feature of the invention;

Figure 3 is a fragmentary enlarged sectional View illustrating a modication of the construction of the plug illustrated in Figure 1 in order to carry out the invention;

Figure 4 is a fragmentary view, partly in section, illustrating the manner in which the spark plug shielding well is insulated against electrical flash-over to ground;

Figure 5 is a view similar to Figure 4 illustrating a modified form of insulating the bottom of the shielding well of the spark plug against flash-over;

' Figure 6 illustrates a modified spark plug construction in which the insulating lining of the shielding wellof the spark plug is made as a unitary part of the central electrode of the spark plug to prevent flash-over from the ground through the lower end of the shielding-well;

Figure 7 is an .illustration of a sealing means for enclosing the contacts of the cable terminal and spark plug electrode to preventvr flashover;

Figure 8 is a fragmentary view, partly in section, illustrating the application of the invention in preventing electrical -ilash-over at the connection of the Spark plug ignition cable to the .distributor block of the magneto.

Referring now to Figure 1 there is illustrated in this ngure a conventional shielded spark plug now widely employed for aviation engine ignition, this plug being illustrated for a better .understanding of the invention. The spark plug has a metallic body I adapted to be screwed into the engine cylinder'and is provided with an integral tubular metal shielding extension 2 threaded at its upper end as at 3 for coupling to the ignition cable shielding harness, not shown. The central electrode l and insulator! are seated within the plug body on copper seal-v ing gaskets 6 maintained in sealing relation by highly compressed powdered ceramicmaterial 1. The interior of the tubular'extension 2 forms a shielding well, which is lined with an insulating ceramic sleeve l, which rests at its lower end on the vinsulating material 1 and at its upper end is clamped in place by a sealing ring 9 retained by a spun-over flange I formed on the upper part of the shielding extension 2'. At high altitudes the pressure within the shielding well is greatly reduced so that the high voltage ignition current passing from the ignition cable conductor to the contact on the upper end of the central electrode I will jump to ground by way of the path of least resistance, which generally occurs within the shielding well, such as though the clearance space i I between the annular ceramic shielding sleeve 8 and the upper end of the insulator and then through the porous insulating material 'l directly to the grounded shell of the plug. A second path for hash-over is upward from the contact on the central electrode 4 along the inner surface o! the insulating sleeve 8, the spark then Jumping on to the grounded flange or lip l! of the tubular shielding extension 2. In either oi' the above cases o1' iiash-over, the plug will fail to fire at the spark gap within the engine cylinder. The plug as so far described is entirely conventional and the construction per se is not a part of my invention. By a very simple means, however, this plug can be so modified as to prevent flashover in the shielding well, as will be hereinafter described. 'I'he flash-over prevention means includes a novel terminal construction described and claimed in my United States Patentlo. 2,280,855, which will now be described as illustrated in Figure 2.

Referring to Figure 2 the terminal for the spark plusr ignition cable Il, which may be,i'or example, of ve millimeter diameter, comprises a tubular rubber protecting sleeve Il, which tightly encompasses the cable and extends upward a distance -sul'iicient to pass through the shielding harness elbow. not shown, and the cable and sleeve being inserted into a ceramic thimble I1 which carries a contact spring at its lower end, which is adapted to engage the contact on a central electrode of the-spark plug and the contact spring i0 being suitably connected to the conductor cable II. A second rubber sleeve Il surrounds the cable and the sleeve abuts the upper end oi' the thimble I1 and extends upward for a distance sumcient to pass through the' elbow or other typeof shielding harness connection. The sleeve il has integrally formed therewith intermediate its ends an annular sealing washer 2lwhich normally seats on the hanged lip Il of the shield extension 2 oftheplugofll'igureltosealthejunctionbetween the shielding well and the ignition harness, but which, however, does not prevent flash-over from the contact on the central electrode upward tothe aforementioned lip Il. In the event that standard seven millimeter ignition cable i# Qmplayed the amer sleeve la is eliminated su: the terminal construction otherwise remains the same.

In accordance with the present invention, as

" illustrated in Figure 3, the plug of Figure l is modied by removing the flange Il so that the sealing washer forms a counterbore Il, which provides a gasket seat Il directly on the upper end of the ceramic lining 8 of the tubular shielding extension 2 oi the plug of Figure l so that the sealing washer 20 (Figure 2) can tightly seat directly on the upper end oi the ceramic insulating sleeve 8 and thus insulate the upper end of the spark plug shielding well against the flashover of high-tension current to the grounded shell 2. It is to be understood that in this construction the sealing washer may trap air at atmospheric pressure within the shielding well and that the invention is not concerned with maintaining atmospheric pressure within the well, but rather to 'employ the sealing washer and the sleeve formed integrally therewith to electrically seal the upper end oi the shielding well from iiash-over, and the term sealing," as referred to the function of the washer 20 (Figure 2). refers primarily to the forming of an insulating barrier to the passage of electric current through the joint between the washer and the gasket seat I4 formed on the upper end of the ceramic sleeve 8.

The assembly of the terminal construction of Figure 2 with the modified form of plug as illustrated in Figure 3 is illustrated in Figure 4. wherein it will be seen that the sealing washer 20 and sleeve I9 eilectively prevent any`passage of electric current upward from spring contact I8 of the cable terminal and contact on the upper end of the central electrode I from reaching the grounded metal shell 2 through the upper end of the shielding well. However, the insulation oi the upper end of the shielding well alone is not suiiicient to prevent ilashover within the well to ground unless the lower end of the shielding well is suitably insulated because otherwise high voltage current may pass downward through the clearance space Ii and through the porous pow-V dered insulating material 1 directly to the grounded body i-of the Aspark plug. In order to insulate the lower end of the shielding well against iiashover one of the commercial corona prooilng materials is used, which is a wax-1ike composition and well known in the art. the material being applied to the bottom ot the shielding well while the -p'lug is heated so that the insulating compound melts andilcws downward, impregnating the-porous insulating material 1 and completely filling the clearance space Il. In laboratory-tests this method of insulating the lower portion of the shielding well has been proven to be elective at pressures equivalent to v altitudes in excess of 50,000 feet.

Obviously the insulating of the lower portion oi' the shielding Vwell against iiash-over may also be accomplished by using a Bakelite varnish, or similar uncured thermo-setting synthetic resin, in place oi' the above-mentioned corona prooling composition, the varnish or uncured resin being forced into placeV under high-pressure and then cured by heat applied simultaneomly with the application of pressure. This mode of imulailng would form an actual bond between the oenh'al .electrode insulator 5 of the spark plug m4) .andtheceramicliningloftheshieldingweil and further would impregnato the powdered ceramic insulation 'I to eilectively prevent flashover through the bottom of the shielding well. In place of the synthetic resin as noted above the clearance space I I may also be filled with a A.vitreous enamel which can be fused at a low enough temperature to avoid damage ofthe plug, and such an' enamel iiller will eiIectively bond the insulator of the central electrode insulator to the lining of the shielding well of the sp'ark plug, and also prevent flash-over to the lower portion of the shielding well. Both the aforementioned methods are illustratedby Figure 5.

A further modified form of plug construction is illustrated in Figure 6 in which the spark plug body 33,V similar to that of the plug of Figure 1,y

is provided with -an annular metal shield 3l threaded as at 432 to receive the coupling member f oi.' the ignition harness. The shield 3i is provided with a threaded .extension 33 adapted to be screwed -into athreaded aperture 34 in the upper portion of the plug-body. A conventional electrode insulator 35 supporting the central .electrode 3.6 is positioned in the 'plug body to lseat the note in upon gaskets -31 which gaskets'are compressed into tight sealingrelationvby the threaded extension 33 of the shielding well.- Central electrode insulator 35 is provided with an annular portion I 38 which is formed integral therewith and which serves asa lining for the shielding'extension 3| of the spark plug, and sincethis lining is made in 'one piece with the insulator, all leakage or flashover/'through the lowerportion of the shielding well is prevented. fAt its upper end the insulating lining 38 is `provided with a gasket seat 40,

which is adapted to receive the gasket 20 of a terminal construction of the same character as illustrated in Figure 2, and whichefiectively prevents the ash-over through the upper enz of the shielding well. As will be noted in .Figure '6, this type of construction permitsthe overall .length of the plug to be appreciably reduced 20 must be such that when the washer 20 is firmly seated in the upper end of the shielding well of the spark plug there will be suilicient compressive force exerted on the sealing member 22 to compressA the spring I3 and bring the `sealing member into rm engagement with the gasket seat 5b.

In addition to the use with spark plugs, the general principle of the inventionis also applicable to prevent flash-over at the 'terminal connection of the spark plug ignition cable to the magneto distributor block and this application is illustrated in Figure 8. As seen in this figure the reference numeral 50 indicates the metallic housing surrounding the distributor block 5l of a .magneto (not shown), the housing 5| being provided with a conventional extension 52 through which the ignition cables pass into the respective cable harness conduits. The distributor block 5i is provided with the usual high-tension contacts 53, each of which is respectively located in a well indicated by the same reference numerals as in the latter noted figure. Each well 54 is provided` with a counterbored portion 55 which at its lower end serves as a gasket seat for the annular gasket 20 of the terminal and threaded at its outer 'end to receive an insulating gland nut 56, Whichholds the sealing washer and .terminal construction inplace when the contact I8 is brought into engagement with a contact 53,' and the gland nut 56 is brought into engagement with the sealing washer which serves as an effective barrier against flash-over `from within the well 54 to the grounded shielding elements of the distributor block. It is also noted that the type'of glandv screws or other clamping means to retain the terminal in electrically sealing engagement in the terminal well will depend upon the construction of the magneto distributor head and shieldbeen illustrated and described it will become apparent to those skilled in the art that other modiiications and alterations may be made therein trated in Figure 1, is'schematically shown in Figure -7, and while this construction is capable of being employed without any additional insulating precautions other than as provided in the, plug of Figure 1, it is preferred that the improved con-- struction be employed with the modifications illustrated in Figuresfi.` 5,*and 6, and it consists of forming the upper end of the central electrode insulator B-of the same character as in the plug of Figure l-with a depression 5a into which the contact of the central electrode I is seated, the

upper end of the insulator A5 being ground to form a nat annular gasket seatV 5b. The thimble lI'l of the terminal construction illustrated in Figure 2 is slightly modified by making the contact spring I8 of smaller diameter so as to nt into the depression 5a of the insulator l5 to engage the contactof the central electrode, and this spring is surrounded by an annuular cup-shaped seal 22 'minal assembly below the annular sealing washer falling within the scope of the invention defined by the appended claims.

I claim:

1. A flash-over proof shielded spark plug and i terminal construction in which the spark plug is of the character having a central electrode exten-ding into a'tubular shielding extension of the plug, the tubular'shielding extension being lined with an insulating material to form a terminal shielding well; the improvement which comprises means for electrically insulating the bottom of the shielding well from electrical dash-over from the central electrode to the grounded spark plug shell, a gasket seat provided on the upper end of the linsulated lining of the shielding Well, asleeve us l of rubber orl like resilient insulating material tightly surrounding the ignition cable and extending into the shielding well and having a length suiiicient to extend beyond the spark plug shield into the shielding harness, and a sealing washer integrally formed with said sleeve intermediate its ends land adapted to be brought into tight sealing engagement with the said gasket seat provided on the insulating lining of the shielding well, whereby electrical flash-over from the central electrode ofthe spark plug to the grounded tubular shielding extension of the plug is prevented.

cally insulating the lower end of the shielding well comprises a bond between the insulating lining of the' shielding well adjacent its lower end and the insulator of the said central electrode.

4. The structure as claimed in claim 1', including an insulator for the spark plug central electrode and in which the insulating lining of the shielding well forms an integral part of said central electrode insulator.

5. A shielding construction for spark plugs comprising a tubular metal shielding extension on the spark plug, an insulating lining in said extension, a central spark plug electrode and' insulator Ytherefor extending within said shielding well, means for electrically insulating the lower end of said shielding well against nashover of current from the central electrode to the grounded metal portions of the plug, a gasket seat provided on the upper end of the insulating lining of the shielding well, an insulating sleeve of resilient material adapted to tightly -encolupass the ignition cable and extending within the shielding well of the plug, and an annular insu lating gasket integrally formed with said sleeve and adapted to be brought into tight engagement with thegasket seat on the said insulating lining of the shielding well.

6. In a shielded spark plug construction of the character in winch a tubular metal shield exten. sion ofthe spark plug is provided with an insulating lining forming a.shielding well for the cable terminal, and the shielding well being insulated at its lower end against electrical leakage-to ground from thecentral electrode of the spark plug; the improvement which comprises a gasket seat on the upper end of the said insulating lining, a gasket of resilient insulating material sealingly engaging the seat on said insulating lining and a sleeve formed integral withsaid washer to tightly encompass the ignition cable extending within said shielding wel-I,where.

by flash-over from the central electrode of .the

spark plug through the upper endof the' shielding well to the grounded shield extension is pre# plug centralelectrode and insulator therefor. are' plug and terminal con.

surrounded by a grounded metal shield which serves as a receptacle forY a cable terminal, the said terminal including a thimble of insulating material having a spring contact thereon adapted to electrically connect the ignition cable to a contact formed on the upper end of the central electrode; the improvement .which comprises an annular gasket seat on the upper end of the central electrode insulator such that the seat extends above the contact on the upper end of the central electrode, and an annular resilient seal bonded to the bottom of the terminal thimble concentric with thev spring contact thereon, whereby when said seal is brought into engagement with the gasket seat on said central electrode insulator flash-over from the central elec-l trode and cable terminal contacts to grounded portions of the plug is prevented.

8. In a shielded spark plug construction of the character wherein the central electrode contact of the plug and the ignition cable terminal are housed in av grounded metallic radio shield open at one end for insertion of the cable terminal and the said shield being lined-with an insulating material to form a shielding weil; the improvement which comprises an insulating barrier at the lower end of the shielding well to Prevent flash-over therethrough from the central electrode contact to grounded metal portions of the plug, and a removable annular insulating seal adapted to engage the insulating lining ofthe shielding well adjacent the open end thereof to prevent flash-over from the central electrodev by a contact on the cable terminal and the mate'- rial of the walls of said well forming an insuiating barrier against electrical leakage from said contact downward through said well to adjacent Qgroundedmetal parts; the improvement which .comprises a gasket seat on the insulating lining oi the shielding well, an annular sealing washer 4 for rubber or like resuient insulatingmatenai adapted to be clamped in tight insulating envgagemeni; with the said gasket seat, and a sleeve formed integrally with said annular sealing -washer and extending interiorly and exteriorly of said shielding well and adapted to tightly encompass the ignition cable extending intosaid shielding well.

JOHN J. ROSE. 

